The nitrogen atom in amines is trivalent and possess an unshared pair of electrons. The geometry of trimethyl amine is …………………… .

The correct option is Pyramidal.

To understand the geometry of trimethylamine, we need to look at the hybridization and bonding of the central nitrogen atom. In trimethylamine, $N{\left(C{H}_3\right)}_3$, the central nitrogen atom is bonded to three methyl ($—C{H}_3$) groups and possesses one unshared pair of electrons (lone pair).

The nitrogen atom undergoes $s{p}^3$ hybridization. This hybridization produces four $s{p}^3$ hybrid orbitals directed towards the corners of a tetrahedron. Three of these hybrid orbitals overlap with the orbitals of the carbon atoms from the three methyl groups to form three $\sigma$ (sigma) bonds. The fourth $s{p}^3$ hybrid orbital contains the lone pair of electrons.

According to Valence Shell Electron Pair Repulsion (VSEPR) theory, the presence of the lone pair leads to lone pair-bond pair repulsions, which are stronger than bond pair-bond pair repulsions. This distorts the regular tetrahedral geometry, compressing the bond angles slightly. Because the molecular geometry only describes the positions of the atoms (and not the lone pair), the shape of the molecule is pyramidal (specifically, trigonal pyramidal).